A photovoltaic generator typically comprises at least one string made of photovoltaic modules connected in series; as a rule it has a plurality of such strings connected in parallel.
If a photovoltaic generator insulated with regard to ground is operated without reference to ground, the system voltage, i.e. the voltage present between the connections of the photovoltaic generator to the inverter, is symmetrically distributed with regard to ground. Then, the voltage relative to ground which is present at one of the photovoltaic modules of the photovoltaic generator is half the system voltage at maximum.
Depending on the construction of the inverter, for example, in case of a galvanic separation in the area of the inverter or in case of using a galvanically separating transformer, it is possible to directly ground one point of the photovoltaic generator or to indirectly predetermine the voltage of this point with regard to ground, i.e. its potential, by suitable means. Thus, for example, the center point of a photovoltaic generator may be grounded to split its system voltage symmetrically with regard to ground. Alternatively, the positive or negative connection of a photovoltaic generator to the inverter may be grounded to keep all of its photovoltaic modules at a negative or positive potential which is an advantage for the lifetime of certain photovoltaic modules, or to asymmetrically predetermine the potential of the center point or of the positive or negative connection with regard to ground, see DE 20 2006 008 936 U1. In case of a ground fault in the area of the photovoltaic generator or its connections to the inverter, this ground reference has to be immediately interrupted to avoid high ground currents.
In case of a ground fault at a photovoltaic generator operated insulated from ground or purposefully separated from ground due to a ground fault, there is the danger that parts of the photovoltaic generator are raised up to the full system voltage relative to ground. This is critical if the system voltage exceeds the insulation design voltage of the photovoltaic modules of the photovoltaic generator. The insulation design voltage is that value of the voltage which may, at maximum, be present between a photovoltaic module of the photovoltaic generator and ground, as the photovoltaic modules do not have a sufficient insulation protection for any higher voltages and/or as it is generally not allowed to exceed this insulation design voltage at the photovoltaic generator according to relevant security regulations.
As the system voltage in operation of a photovoltaic generator does not exceed the open-circuit voltage of the photovoltaic generator, the danger of surge voltages exceeding the insulation design voltage of the photovoltaic modules of the photovoltaic generator only exists if the photovoltaic generator has a higher open-circuit voltage than the insulation design voltage of its photovoltaic modules. Vice versa, this danger is in fact present in each photovoltaic generator whose open-circuit voltage exceeds the insulation design voltage of its photovoltaic modules, because the photovoltaic generator may be in open-circuit operation when a ground fault occurs.
Comparatively high system voltages at the DC voltage side of an inverter are an advantage in feeding electric energy from a photovoltaic generator into an AC power grid as they limit the currents necessarily flowing for transferring a certain electric power from the photovoltaic generator to the inverter and to thus limit the line cross-sections required for conducting these currents.
In case of a known method of protecting a photovoltaic generator in case of a ground fault, the photovoltaic generator, in case of the ground fault, is short circuited between its connections to the inverter via a contactor. A security device suitable for this method is known from WO 2011/135239 A1. The contactor has to be of a sufficiently big dimension to cope with high currents, particularly if the photovoltaic generator comprises a plurality of strings connected in parallel and made of photovoltaic modules connected in series. In this case, the short-circuit current may be much higher than the nominal current from the photovoltaic generator. As a result, the contactor for short circuiting the photovoltaic generator becomes quite expensive.
For the purpose of limiting a photovoltaic voltage to a value allowable for a connected load, it is known from DE 30 41 078 A1 to periodically temporarily short circuit a part of the photovoltaic generator providing the photovoltaic voltage, i.e., a some of its photovoltaic modules, by means of a switch. Here, the photovoltaic voltage not bypassed by the switch is always smaller than or equal to the maximum allowable or nominal voltage of the connected devices. The switch for short circuiting a part of the photovoltaic generator is arranged at the location of the photovoltaic generator. If the photovoltaic generator comprises a plurality of strings of photovoltaic modules connected in parallel, one switch for partially short circuiting has to be provided for each string.
As a further development of the teaching of DE 30 41 078 A1, it is known from DE 10 2010 009 120 B4 to protect a large-sized photovoltaic power plant against surge voltages in case of no AC current being fed in that a short-circuit switch by which a part of the photovoltaic modules of the respective string is short circuited is only provided with some of its strings connected in parallel. In this way, it is intended to also reduce the potential of the strings without short-circuit switches to the lower potential and thus to a non-dangerous value. Here, a reduced number of switches but still some switches are used outside the inverter via which electric power is fed into the AC power grid. Further, it is hardly sufficient to only short circuit parts of few strings of a photovoltaic generator as there is otherwise the danger that the system voltage is not reduced in this way but a backflow current through the partially short circuited strings is caused instead, if this is not avoided by means of additional measures further increasing the constructional efforts for this solution.
From EP 2 284 973 A1 it is generally known that a photovoltaic generator may be short circuited via the semiconductor switches of an inverter bridge of an inverter to which the photovoltaic generator is connected for feeding electric energy into an AC power grid, after an input side intermediate link capacitance of the inverter has been discharged via a resistor. Here, the photovoltaic generator is short circuited to remove the driving force of a backflow current which has been registered with one of the strings of the photovoltaic generator connected in parallel. This registration takes place via current sensors which are assigned to each one of the individual strings and whose signals are supplied to the inverter from the outside. The photovoltaic generator is not surveyed for a ground fault, and any ground fault is also not indirectly registered in the form of backflow currents, as at least simple ground faults do not cause such backflow currents. EP 2 284 973 A1 does also not deal with keeping an insulation design voltage of photovoltaic modules of the photovoltaic generator.
There still is a need of a method for protecting photovoltaic modules of a photovoltaic generator against surge voltages relative to ground in case of a ground fault, which may be implemented in an inverter for feeding electric energy from the photovoltaic generator into an AC power grid without high constructional effort and costly components.